21 : 
“ The cause of this neglect is perhaps the severe criticism 
of it which Newton made at the time of its invention: as com- 
pared to his telescope it is in some respects inferior, though 
on other grounds than those commonly assigned ; but in others 
it issuperior. The defects are— 
“© 1. In large instruments it is difficult to keep the magnify- 
ing power sufficiently low, the second image being five or six 
times as large as what is due to the focal length of the great 
speculum; and this is essential, because the air is seldom so 
calm as to admit, with large apertures, of using powers propor- 
tional to those of lesser instruments. 
«<2. The rays have to pass thrice the length of the tube; 
while in the equivalent Newtonian it is twice the length: the 
tube is, however, shorter in the first instance, so that the ac- 
tual spaces are as 1:1:28. As there is almost invariably a 
difference of temperature between the great speculum and the 
air in the tube, the latter is affected with eddies and currents, 
which cause indistinct vision in proportion to the quantity of 
disturbed medium which the rays traverse. 
«« 3. The small mirror is larger than in the Newtonian, 
and therefore stops a little more of the central rays. 
‘¢ It was also objected by Newton that the field of view 
must be small, and that the reflection is more intense in the 
incidence of 45 on his small mirror, than at the perpen- 
dicular. é, 
«< Of these the greater length of the light path is the only 
valid objection, and even this may probably be made less po- 
tent by establishing a downward current in the tube, or by 
apertures in it, to let the heated air escape freely. The mag- 
nifying power can be such as just to allow of the eye taking 
in the whole pencil, below which one cannot go without 
giving up part of the advantage of an instrument’s size. The 
field can be quite as large asin the Newtonian. The loss of 
light by the difference of the small mirror is insignificant, the 
more so as the central rays are really the least valuable of all: 
